This invention relates to an austenite stainless steel having an excellent high temperature strength and suitable for use to manufacture boilers, steam turbines, chemical plants and nuclear plants.
Usually, low alloy steel is used as heat resistant steel used at a temperature of less than 600.degree. C., and 18-8 stainless steel is used for temperature above 600.degree. C. However, at temperature above 700.degree. C., the mechanical strength of stainless steel is not sufficiently high so that it can not be used as structural members operating at temperature higher than 700.degree. C. Typical structural members operating at temperature higher than 700.degree. C. include a centrifugal cast tube of HK-40 (25Cr-20Ni-0.4C) and nickel base alloys. However, when these alloys are rolled their strength decreases and they are expensive. Although alloys whose strength is increased by dispersing oxides have also been proposed, they are not only expensive but also difficult to weld. Furthermore, although titanium or niobium was incorporated together with a large quantity of carbon or nitrogen for the purpose of obtaining high strength alloy at low cost, each of such alloy has a defect that its stiffness decreases greatly during use. For use at a temperature lower than the low oxidation potential of a high temperature gas furnace. Hastelloy X or other nickel base super alloys have been investigated, but these alloys are extremely expensive so that it is parctically impossible to use them to construct nuclear power plants or other structures.